Several solutions are known in the art to reduce or eliminate the combustibility of polymers. Some of such solutions are based on the use of metal compounds, in particular of antimony, bismuth, or arsenic, in combination with partially halogenated, thermally unstable organic compounds, such as chlorinated paraffinic waxes.
Other solutions are based on the use of substances capable of yielding intumescence. The formulations of intumescent type are generally constituted by the polymer and at least three main additives: an essentially phosphorus-containing additive, whose purpose is of forming, during the combustion, an impermeable, semi-solid vitreous layer, essentially constituted by polyphosphoric acid, and of activating the process of formation of intumescence; a second additive, containing nitrogen, which performs the functions of a foaming agent; and a third, carbon-containing additive, which acts as a carbon donor to allow an insulating cellular carbonaceous layer ("char") to be formed between the polymer and the flame.
Examples of intumescent formulations of this type are those reported by the following patents: U.S. Pat. No. 3,810,862 (Phillips Petroleum Co.) based on melamine, pentaerythritol and ammonium polyphosphate; U.S. Pat. No. 4,727,102 (Vamp S.r.1.), based on melamine cyanurate, a hydroxyalkyl derivative of isocyanuric acid and ammonium polyphosphate; and by published patent application WO-85/05,626 (Plascoat U.K. Limited), on the basis of various phosphorus and nitrogen compounds among which, in particular, a combination of melamine phosphate pentaerythritol and ammonium polyphosphate has to be cited.
In more recent formulations, together with the use of an organic or inorganic phosphorus compound a nitrogen-containing organic compound was used, generally consisting of an amino plastic resin obtained by condensation of urea, melamine or dicyandiamide with formaldehyde.
Examples of formulations containing two additives are those reported in the following patents: U.S. Pat. No. 4,504,610 (Montedison S.p.A.) based on oligomeric derivatives of 1,3,5-triazine and ammonium polyphosphate and European Patent No. 14,463 (Montedison S.p.A.) based on organic compounds selected from among benzylguanamine and reaction products between aldehydes and several nitrogenous cyclic compounds, in particular benzylguanamine-formaldehyde copolymers, and ammonium polyphosphate.
Self-extinguishing compositions can be also obtained by using single-component additives, which contain in their organic molecule both nitrogen and phosphorus atoms, as disclosed in U.S. Pat. No. 4,201,705 (Borg-Wagner Corp.).
These intumescent flame retardant systems endow the polymer, which contain them, with the property of forming a carbonaceous residue when they undergo a fire, or are exposed to a flame. This kind of flame-retardant systems display a large number of advantages: absence of phenomena of corrosion in the machinery in which the polymers are processed, a lower emission of smoke as compared to the systems containing metal compounds and halogenated hydrocarbons, and, above all, the possibility of endowing the polymers with satisfactory flame-proof properties, with a smaller amount of total additive and therefore without excessively impairing the mechanical properties of the same polymers.